Fixing cppcheck warnings.

jerry-libc/jerry-libc-defs.h

@@ -41,7 +41,7 @@ extern void __attr_noreturn___
 libc_fatal (const char *msg,
             const char *file_name,
             const char *function_name,
-            const int line_number);
+            const unsigned int line_number);
 
 #ifndef LIBC_NDEBUG
 # define LIBC_ASSERT(x) do { if (__builtin_expect (!(x), 0)) { \

jerry-libc/jerry-libc-fatals.c

@@ -30,7 +30,7 @@ void __attr_noreturn___
 libc_fatal (const char *msg, /**< fatal error description */
             const char *file_name, /**< file name */
             const char *function_name, /**< function name */
-            const int line_number) /**< line number */
+            const unsigned int line_number) /**< line number */
 {
   if (msg != NULL
       && file_name != NULL